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The volunteers, who ranged in age from 19 to 36, were monitored in an fMRI machine as they were shown one of the words along with a colored signal. On a green signal, the volunteers were asked to recall the word and its pair — keyboard and winter, for example. On a red signal, they were told to recall the first word but stop themselves from recalling its pair.

The study results showed that people who were best at blocking the unwanted thoughts — in this case the the word pair — also had the highest concentrations of an important chemical messenger called GABA in their brain's hippocampus (that's the horseshoe-shaped region that's key in forming memories and managing emotions).

GABA, or gamma-aminobutyric acid, is often thought of as a sort of "downer" chemical because of its tendency to tamp down on hyperactive brain cells. The chemical may play a role in controlling our fear or anxiety response, and several studies have suggested a link between low GABA levels and various psychiatric illnesses including depression, schizophrenia, and anxiety.

"What's exciting about this is that now we're getting very specific," Michael Anderson, a senior scientist and program leader at the University of Cambridge's neuroscience department, told the BBC. "Before, we could only say 'this part of the brain acts on that part,' but now we can say which neurotransmitters are likely to be important."

Still, the study has some limitations in terms of its applications for people with psychiatric diseases. The researchers only studied cognitively healthy young adults, not people with mental illness, and only asked them to recall simple verbal items, as opposed to complex thoughts. It's also possible that the thoughts that could plague someone with depression are significantly different from those of a healthy person. (Where a healthy person might think, 'I shouldn't have skipped the gym today,' for example, a depressed person might take that to an extreme like, 'I'm worthless.')

For those and other reasons, the researchers acknowledge that their current findings aren't enough to say whether a negative thought from a depressed person relies on GABA the same way it does in a healthy person. Nevertheless, dozens of studies suggest the chemical is an important clue for future research.